The present invention relates to a high-low speed two step speed control apparatus for an AC motor which is suitable for stopping a load operated by the AC motor at a predetermined position. More particularly, it relates to a speed control circuit which easily changes from high speed operation to low speed operation in the AC motor for low speed operation under low frequency output given by the phase control using thyristors.
There has been proposed a system for obtaining approximate low frequency outputs with partial deletion of wave forms of an AC power source by using thyristors. In this system, it is difficult to change from full speed operation to low speed operation without inserting a smoothing reactor having a high capacity depending upon the motor because of the high frequency components in the low frequency output. In particular, when the load of the AC motor is light, the full speed rotation of the motor is maintained even though a low speed mode is given.
The inventors have found that the above-mentioned difficulty can be overcome by exchanging the order of phases to the full speed operation in the low speed operation. A purpose of the present invention is to eliminate the need for synchronization under this condition. Another aim of the invention is to provide braking control apparatus for rapidly stopping a motor such as a three-phase cage type induction motor by feeding DC current to the primary windings and for accurately stopping the motor in a relatively low speed operation under speed control by a predetermined frequency by using switches such as thyristors, transistors, etc.
In general, in order to control the stop position for a machine operated by a motor, it has been proposed that the speed reducing operation be fluctuated after the high speed operation is corrected in low speed operation and to then suddenly stop the motor. It is known to conduct a low speed operation with frequency control of an AC motor. On the other hand, it is also known to suddenly stop the motor by applying DC voltage to the primary windings of the AC motor instead of the AC voltage.
However, in the conventional DC excitation braking operation, DC voltage is applied to a desirable one phase or two phases of a three phase induction motor without any consideration of selection of phases. The invention concerns the selection of phases for applying DC voltage in order to improve accuracy in stopping the motor. The selection is dependent upon the phase and the polarity of the voltage applied at the moment just before the initiation of DC braking operation.
Thus, a purpose of the invention is to provide a speed control apparatus for an AC motor using frequency control. More particularly, it relates to a speed control apparatus for accomplishing stable high-low two step speed operation at high efficiency by the partial deletion of the phases of the commercial AC power source.
In general, there is a primary voltage control system using phase control and a frequency control system using an inverter as a speed control system for an induction motor by using control rectifiers such as thyristors. The former system has a simple structure in principle. However, feedback of speed is necessary and efficiency is low. The latter system can yield a stable speed without feedback of speed to operate at high efficiency. However, the apparatus is disadvantageously expensive.
Accordingly, it is considered to realize low speed operation by obtaining low frequency current using partial deletion of the phases of a commercial AC power source in utilizations for inexpensive high-low two step speed operation without the requirement of continuous speed control such as a stop operation at a predetermined position for a conveyer, an electric truck, a turn-table and the like, a calculating machine, a stepping machine, a screw conveyer, a constant volume control and for constant distant movements of various automatic assembling machines.
In these control apparatus, the turn-on control of the control rectifiers is conducted so as to apply the interphase voltage of the AC power source by selecting in a predetermined order the interphase corresponding to the motor. In the conventional apparatus, the output signals of the control apparatus are prevented until the synchronizing condition is accomplished between the phase of the AC power source and the predetermined selected signal of the control apparatus after the connection of the power source. In some instances, uncontrolled signals are fed out.
The invention provides a control apparatus of a motor for outputting normal signals at the time connection is made to the power source without these disadvantages.
Moreover, it is known to conduct a low speed operation by converting frequency applied to the AC motor under a control using thyristors. The control modes of the control apparatus include 1 a mode for full speed operation during turn-on of the thyristors; 2 a mode for low speed operation under the control of turn-on of the thyristors and 3 a mode for braking the operation with a DC output using turn-on of a thyristor disposed in one specific phase to one direction.
In general, it is necessary to have means for receiving a mode change command given from the outer part of the control apparatus and electrical insulation between the control apparatus. A purpose of the invention is to provide a speed control apparatus which can receive an effective mode change command for applications for the modes 1 and 2 or modes 2 and 3 or modes 1 2 and 3 . The invention also relates to a speed control apparatus for the AC motor.
The conventional speed control apparatus include mechanical gears and a polarity change motor or an NC motor. However, the first and second type of apparatus have disadvantages in that they require a large and complicated structure of large volume. The second and third apparatus have the disadvantage of an expensive cost. An aim of the invention is to provide small size and inexpensive speed control apparatus which can be combined with the usual AC motor.